Speaker of the House Paul Ryan and House Majority Whip Steve Scalise arrive to discuss tax reform during a Tuesday press conference. “When we rolled the tax bill out last week, we were completely unified,” Scalise boasted in an interview that day. | Saul Loeb/AFP/Getty Images

House GOP leaders are using a mix of behind-the-scenes cajoling and warnings about losing the majority to corral their oft-fractured conference on tax reform.

House Majority Whip Steve Scalise’s team has been pulling district-by-district data to prove to skeptical lawmakers from high-tax states that their constituents will see a tax cutunder the plan. That strategy successfully flipped Rep. Tom MacArthur (R-N.J.) from a “no” last week to a “lean-yes” over the weekend.

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But leadership sources say the toughest part of the whipping effort has yet to begin. That's because on Thursday, the Senate will release its own tax bill that is expected to upend some of the House's careful negotiating. The Ways and Means Committee will also finish consideration of the bill this week, slamming the door shut on any last-minute changes sought by Republicans from across the conference.

That’s when the real battle begins for GOP leaders, as lawmakers’ last opportunity to save their pet tax breaks fades away.

“There’s always a flurry of activity once you have a final product, but I believe that when we get back here next week, we will be here until we pass our tax reform,” said chief deputy whip Patrick McHenry (R-N.C.). “The toughest part about any whipped bill is the finally flurry of action, always.”

GOP leadership will whip the tax bill early next week to determine how many votes they still need. They’re also discussing whether to hold the chamber in session until the tax bill passes — perhaps even through the weekend.

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Leaders have their work cut out for them. While they won MacArthur, they just lost Rep. Darrell Issa (R-Calif.), whose defection on Tuesday shocked senior Republicans.

Issa and several New York Republicans are pushing to restore the state and local tax deduction frequently claimed by their constituents. Rep. Dan Donovan, another such holdout, said he and fellow Empire State Reps. Peter King and Lee Zeldin have submitted requested changes on the matter to Ways and Means.

Donovan won’t say how he will vote on the measure until he sees the panel's final product — and whether his changes were accepted: “After they close out that markup, that’s when we’ll figure out if the people of New York benefit or not.”

The New Yorkers aren’t the only ones buttonholing tax-writers to tweak the bill to their liking. Republican Study Committee Chairman Mark Walker (R-N.C.) told POLITICO he’s been trying to convince a few tax Ways and Means membersto offer amendments retaining the adoption tax credit, which he called a “pro-life” issue, and make changes to charitable giving benefits.

House Freedom Caucus leaders are also trying to get tax writers to include a controversialrepeal of Obamacare’s individual mandate in the bill. They used a weekly Tuesday luncheon with Speaker Paul Ryan (R-Wis.), a skeptic of the idea, to argue that the more than $300 billion in savings could pay for tax tweaks aimed at winning over other members of the conference.

“We’re hoping these changes happen and the mandate goes in,” Rep. Jim Jordan (R-Ohio) said. “That’s what we told the voters we’d do and it would help a whole host of issues.”

What's unclear is how these members will respond when the markup is finished iftheir changes aren’t accepted. Do they draw a line in the sand and threaten to vote against the bill? Or drop their demands and support legislation Republicans feel is critical for keeping their majority?

GOP leaders are banking on the latter. Still, they’ve been “pre-gaming” for the whipping effort for weeks, as McHenry called it Wednesday.

In an interview in his whip office Tuesday, Scalise argued that the vote-count is in good shape because of the success of the bill's rollout. The Louisiana Republican noted that while their Obamacare repeal bill didn’t have the full support of the president, the conference and outside groups, the tax bill largely does.

Scalise said top Donald Trump aide Kellyanne Conway, for example, came to speak to his deputy whips just this week, sharing statistics with them about how to message the bill.

“When we rolled the tax bill out last week, we were completely unified,” Scalise boasted. “Our members know what’s at stake and how important it is for the country, but then having the president invite us to the White House and say, 'I’m behind this bill 100 percent' … it was probably the best rollout I’ve seen.”

Leadership has also been keeping the heat on members by framing passage as a make-or-break moment to save their majority. After Democrats' big wins races across the country on Tuesday night, Ryan told the Washington Examiner Wednesday morning that passing the tax bill is more important now than ever before.

“We have a promise to keep and we've got to get on with keeping our promise," Ryan said. “If anything, this just puts more pressure on making sure we follow through… we've got to deliver.”

Scalise and McHenry are hoping the district-by-district data, broken into different income levels, will convince skeptics to come aboard. House Majority Leader Kevin McCarthy's (R-Calif.) team has been working to create customized data models so leadership can tweak them for each district and share them with members to win support.

While nothing repels lawmakers faster than a tax hike on their constituents, they're betting they can convince lawmakers that won't be the case.

That's exactly what happened with MacArthur. Last Thursday, MacArthur decried a provision in the tax bill aimed at mollifying supporters of the state and local tax deduction — a property tax deduction capped at $10,000 — as “too low.”

“I’ve done the math for my own state, my own district and I’ve given the chairman of Ways and Means what I think the number needs to be.”

He changed his tune by Sunday.

“We got a huge win by getting this kicked up to $10,000!” he told POLITICO in a Sunday interview, noting that 151,000 people in his district claimed property tax deductions and that only 3,000 in his district were above that $10,000 number. “It covers nearly everybody in my state and in my district.”

“It sure is helpful in educating members about how good this bill is going to be for their districts," Scalise said, even as non-partisan reports showed some people would indeed see higher taxes.

Despite winning MacArthur over, signs are emerging that the guardians of the state and local tax deduction could be further agitated over the release of the Senate tax bill, expected Thursday. That legislation will ax the deduction entirely, throwing out House leaders’ middle-ground proposal for high-tax members.

That could repel state and local-tax cheerleaders from supporting the House bill,for fear the Senate would ultimately jam them.

Others are also leery of the fact that the Senate bill is likely to keep the mortgage interest deduction at $1 million instead of chopping it to $500,000; will offer a larger child tax credit; and could potentially even have a lower bottom rate than the House's tax plan. Many would rather vote for a bill with those items than the one in the House.

“They’ll compare and contrast with the Senate bill, so the minute the Senate bill comes out, people will say, ‘I like that better,’ or ‘I like this better’ so it’s going to create a back-and-forth,” Freedom Caucus leader Mark Meadows (R-N.C.) predicted.

Despite the difficulties ahead, GOP leaders feel they have momentum on the tax bill simply because the of the political pressure to get it done. Republicans by and large believe their majority is at stake, and if they fail, they could lose the House.

“Tax reform’s gone from a policy prerogative to a political prerogative as well,” Rep. Mark Sanford (R-S.C.) told POLITICO. “And so I think there is a desire… to get something done regardless of some of the fine print, some of the detail, based on the belief that people in the Republican Conference think that there will be electoral consequences for themselves if something doesn’t happen.”

Sanford added: “And there’s no motivation like self-motivation in the world of politics.”